Internal combustion type engines employ a crankshaft to drive an apparatus to which the engine is connected, typically a vehicle of some sort, and a cam shaft to regulate the opening and closing of intake and exhaust port valves in the engine. Normally, sprocket and chain type timing means are employed in e.g. automobile engines to interconnect the crankshaft with the cam shaft although interengaging teeth are also used. The timing of the opening and closing of the intake and exhaust port valves relative to the position of the pistons of the engine is extremely critical. Adjustment of valve timing represented by, e.g., 1.degree. or 2.degree. of rotation of the crankshaft relative to the cam shaft can change the character of the engine.
It is desirable to advance or retard the timing of an engine in order to change the power vs. rpm range of the engine. For example, one would wish to advance the timing in order to shift the peak torque range of a particular engine to a lower rpm setting. This would be advantageous, e.g., for trucks, off-road vehicles and the like. In contrast, one would wish to retard the timing in order to shift the peak torque to a higher rpm setting. This may be advantageous for race cars, race motorcycles and the like. In general, it is desirable to have the ability to adjust the timing of engines for various types of vehicles such as trucks, passenger cars, off-road vehicles, motorcycles, snowmobiles, airplane engines, boat engines or even lawnmower engines.
Adjustment of the relative position of the crankshaft and the cam shaft in relation to each other is thus sometimes necessary. Such adjustment frequently requires that the shafts be adjusted circumferentially a distance equal to a fraction of the circumferential extent of one tooth of one of the gears or a distance equal to the circumferential extent of one or a multiplicity of teeth plus a fraction of the circumferential extent of one tooth. Where gears are subject to little force and abuse, they can be fixed to their shafts by lock bolts, set screws or the like so that any desired amount of turning and adjustment of the gears on their related shafts can be effected. However, in heavy duty use, such as in automobile engines and the like, the use of such fasteners is inadequate. Accordingly, keys and keyways are provided in this type of environment to provide accurate indexing of the gears and to provide a secure driving engagement between a gear and its shaft. However, with the use of keys and keyways, difficulty has been experienced in adjusting the timing for relative rotative position of the gear shafts other than a distance equal to the circumferential extent of one or a number of teeth of one of the gears. In such cases where the adjustment needs to be less than the circumferential extent of one tooth, a new keyway has to be cut into the gear in order to so adjust the gear in relation to its shaft.
In one known improvement on a gear having only a single secondary keyway and in which gear new keyways need to be cut on an as needed basis, a gear is known which has a plurality of circumferentially spaced secondary keyways such that a key of the associated shaft can be engaged in a selected one of the keyways in order to effect the desired rotative adjustment between the shaft and the gear. In this known type of gear, the secondary keyways are spaced circumferentially apart a distance equal to the circumferential extent of a predetermined number of teeth on the gear plus a predetermined incremental distance of adjustment effected by rotative movement of one gear a distance from one of the keyways to the next adjacent keyway and aligning to the mating gear by aligning the appropriately marked tooth.
However, one of the difficulties with this known gear is that indicia need to be provided on the face of the gear, typically by stamping, to distinguish one keyway from another. The stamping process adds a manufacturing step which, of course, increases production costs. In addition, by having to label the several keyways on the gear (as well as selected teeth) with symbols, the gear face will contain a sizeable number of symbols which may tend to confuse the mechanic due to the gear's cluttered appearance.
Accordingly, it has been considered desirable to develop a new and improved timing gear device which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.